There are no therapies of demonstrated effectiveness to treat men with osteoporosis. Although osteoporosis is more common in women, about 1/3 of all hip fractures occur in men. In men over age 65, the risk of hip fracture is over half the rate of similarly aged women. Thus, treatments for men with osteoporosis are clearly needed. Further, the contrast between the effects of PTH in men and in women may help illuminate the mechanism of the anabolic effect of PTH. Specific Aim I: To determine whether an initial stimulation of bone resorption is necessary for all or part of the subsequent anabolic actions of PTH on bone in men. Patients with osteoporosis will be treated with calcium and vitamin D for 6 months and then randomly assigned to receive in addition either l) PTH alone, 2) alendronate alone or 3) PTH and alendronate. Biochemical markers of bone formation and resorption will be followed. Specific Aim II: To determine whether suppression of bone resorption enhances the anabolic effect of PTH on bone in men. The major end point will be changes in bone mineral density of the lumbar spine, proximal femur, radial shaft, and total body. These determinations will be made on the three treatment groups described in Specific Aim I. These studies parallel those being conducted in women. We will be able to determine whether gonadal steroids influence the anabolic response to PTH.